The AGILE monitoring of Cygnus X-3: transient gamma-ray emission and spectral constraints

• Published in: Astronomy and astrophysics (Berlin) Vol. 545; p. A110
• Main Authors: Piano, G., Tavani, M., Vittorini, V., Trois, A., Giuliani, A., Bulgarelli, A., Evangelista, Y., Coppi, P., Del Monte, E., Sabatini, S., Striani, E., Donnarumma, I., Hannikainen, D., Koljonen, K. I. I., McCollough, M., Pooley, G., Trushkin, S., Zanin, R., Barbiellini, G., Cardillo, M., Cattaneo, P. W., Chen, A. W., Colafrancesco, S., Feroci, M., Fuschino, F., Giusti, M., Longo, F., Morselli, A., Pellizzoni, A., Pittori, C., Pucella, G., Rapisarda, M., Rappoldi, A., Soffitta, P., Trifoglio, M., Vercellone, S., Verrecchia, F.
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.09.2012
• Subjects: gamma rays: general; outflows; radiation mechanisms: non-thermal; radio continuum: general; stars: individual: Cygnus X-3; stars: winds; X-rays: binaries
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201219145

We present the AGILE-GRID (Astro-rivelatore Gamma a Immagini LEggero – Gamma-Ray Imaging Detector) monitoring of Cygnus X-3, during the period between November 2007 and July 2009. We report here the whole AGILE-GRID monitoring of Cygnus X-3 in the AGILE “pointing” mode data-taking, to confirm that the γ-ray activity coincides with the same repetitive pattern of multiwavelength emission and analyze in depth the overall γ-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense γ-ray events were detected in this period, with a typical event lasting one or two days. These durations are longer than the likely cooling times of the γ-ray emitting particles, implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid that then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with both X-ray and radio monitoring data, we find that the main events of γ-ray activity were detected while the system was in soft spectral X-ray states (RXTE/ASM (Rossi X-ray Timing Explorer/All-Sky Monitor)count rate in the 3−5 keV band  ≳ 3 counts s-1), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT (Burst Alert Telescope) count rate ≲0.02 counts cm-2 s-1), a few days before intense radio outbursts. This repetitive temporal coincidence between the γ-ray transient emission and spectral state changes of the source turns out to be the spectral signature of γ-ray activity from this microquasar. These γ-ray events may thus reflect a sharp transition in the structure of the accretion disk and its corona, which leads to a rebirth of the microquasar jet and subsequent enhanced activity in the radio band. The γ-ray differential spectrum of Cygnus X-3 (100 MeV–3 GeV), which was obtained by averaging the data collected by the AGILE-GRID during the γ-ray events, is consistent with a power law of photon index α = 2.0±0.2. Finally, we examine leptonic and hadronic emission models for the γ-ray events and find that both scenarios are valid. In the leptonic model – based on inverse Compton scatterings of mildly relativistic electrons on soft photons from both the Wolf-Rayet companion star and the accretion disk – the emitting particles may also contribute to the overall hard X-ray spectrum, possibly explaining the hard non-thermal power-law tail seen during special soft X-ray states in Cygnus X-3.